I shot stuff for someone who wants to review SxS footage on his G5 running 10.4.11. He just emailed saying that it won't install on his machine. Is there a version that will, or does he need to upgrade to Leopard?

Clip Browser 2.6 does support Mac OS 10.4.11, however it only does so on Intel Processors. The G5 PPC cannot be upgraded to Apple's Leopard or Snow Leopard because they only support Intel Processors. I once had a G5 PPC-Quad until I purchased my EX3.

As I said, it will play EX MP4 directly. It doesn't even need the BPAV.
Of course they may want the BPAV for other reasons but for playback of MP4 only, VLC should work. Keep in mind the G5 isn't very zippy compared to Intel Macs. It's simply enough to test though. It requires no codec install.